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It forms above the mountain range, usually at the beginning of a chinook wind as a result of orographic lifting over the range. It appears when seen from downwind to form an arch over the mountain range. A layer of clear air separates it from the mountain. [3] A view of the Front Range of the Rockies capped by a föhn wall.
Wind and moist air are drawn by the prevailing winds towards the top of the mountains, condensing and precipitating before it crosses the top. In an effect opposite that of orographic lift, the air, without much moisture left, advances behind the mountains, creating a drier side called the "rain shadow". [citation needed]
An orographic map of Eastern Siberia from 1875 by Peter Kropotkin. Orography is the study of the topographic relief of mountains, [1] and can more broadly include hills, and any part of a region's elevated terrain. [2] Orography (also known as oreography, orology, or oreology) falls within the broader discipline of geomorphology. [3]
Tectonic–climatic interaction is the interrelationship between tectonic processes and the climate system. The tectonic processes in question include orogenesis, volcanism, and erosion, while relevant climatic processes include atmospheric circulation, orographic lift, monsoon circulation and the rain shadow effect.
Orographic or relief rainfall is caused when masses of air are forced up the side of elevated land formations, such as large mountains or plateaus (often referred to as an upslope effect). The lift of the air up the side of the mountain results in adiabatic cooling with altitude, and ultimately condensation and precipitation.
Effect of a rain shadow The Tibetan Plateau (center), perhaps the best example of a rain shadow. Rainfalls from the southern South Asian monsoon do not make it far past the Himalayas (seen by the snow line at the bottom), leading to an arid climate on the leeward (north) side of the mountain range and the desertification of the Tarim Basin (top).
On the leeward side of mountains, desert climates can exist due to the dry air caused by compressional heating. The movement of the monsoon trough, or Intertropical Convergence Zone, brings rainy seasons to savannah climes. The urban heat island effect leads to increased rainfall, both in amounts and intensity, downwind of cities.
The highlands generate orographic precipitation, and are generally cooler and more humid than the lower-elevation deserts and dry shrublands that bound the highlands on the east and south. Orographic effects create fog and cloud cover which keep humidity high and help sustain forests. Southwesterly winds bring rainfall from May to October.